1. Field of the Invention
The present invention relates to a photosensitive device for outputting a one- or two-dimensional image as a digital signal.
2. Related Background Art
A photosensitive device such as a solid-state imaging device or the like has an array of a plurality of photosensitive elements, each of which outputs a signal voltage corresponding to the intensity of incident light. Some solid-state imaging device converts the signal voltage as an analog signal into a digital signal (by A/D conversion), and outputs the digital signal. If a signal voltage exceeds a predetermined value upon A/D conversion, a digital signal to be output, which is obtained by A/D conversion based on that signal voltage, is saturated to a value corresponding to the predetermined value and, as a result, an accurate sensed image cannot be obtained. Hence, conventionally, a signal voltage equal to or more than its expected maximum value is set as the predetermined value to prevent such saturation. Also, the dynamic range is increased using a technique such as logarithmic compression or the like.
The solid-state imaging device is used in, e.g., a active distance measurement device built in a camera. In this distance measurement device, two solid-state imaging devices sense an image of spot light which is projected from a light-emitting diode LEDn or the like toward an object, and is reflected by the object, and distance measurement is done based on the two sensed images. At this time, since the image with the spot light is superposed spot light components upon background light components, the two solid-state imaging device sense only background light components while no spot light is projected, and the difference between the images with and without the spot light components is calculated to obtain images of only the spot light components, thus improving the distance measurement precision.
However, in A/D conversion in the conventional photosensitive device, since a large value is set as the predetermined value to prevent saturation, when the intensity of incident light on each photodetective element is small, i.e., when the signal voltage value is small, the resolution of a digital signal output impairs.
Furthermore, the following problem is posed when an image of only spot light components is obtained by subtracting the image sensing result of background light components from that of the spot light components and background light components like in a case wherein the photosensitive device is used in the distance measurement device. That is, when the background light components are larger than the spot light components, the signal voltage output upon detecting the spot light superposed with the background light components becomes very large and, hence, a large value must be set as the predetermined value to prevent saturation. Therefore, the resolution of a digital signal output based on the spotlight components obtained as the difference further impairs.
The present invention has been made to solve the aforementioned problems, and has as its object to provide a photosensitive imaging device which is free from saturation even when the intensity of incident light is large, and can assure high A/D conversion resolution even when the intensity of incident light is small.
A photosensitive device of the present invention is characterized by comprising (1) a one- or two-dimensional array of N (Nxe2x89xa72) photosensitive elements for respectively outputting signal currents corresponding to the amount of light they receive, (2) N first integrating circuits which are arranged in correspondence with the N photosensitive elements, integrate charges in correspondence with the signal currents output from the photosensitive elements, and output signal voltages, (3) a first maximum value detection circuit for detecting a maximum value of the signal voltages output from the N integrating circuits, and (4) an A/D conversion circuit for setting an A/D conversion range on the basis of the maximum value detected by the first maximum value detection circuit, converting the signal voltages output from the N first integrating circuits into digital signals, and outputting the digital signals.
The photosensitive device comprises N (Nxe2x89xa72) sets of photosensitive elements and first integrating circuits. Each first integrating circuit integrates a charge in correspondence with a signal current which is output from each photosensitive element in correspondence with the intensity of incident light, and outputs a signal voltage. The first maximum value detection circuit detects the maximum value of the signal voltages output from the N first integrating circuits. The A/D conversion circuit sets the A/D conversion range on the basis of the maximum value detected by the first maximum value detection circuit, converts the signal voltages output from the N first integrating circuits into digital signals, and outputs the digital signals. Note that a signal to be selected and detected by the first maximum value detection circuit may be the second largest numerical value in place of the maximum value, or a numerical value of an appropriate order may be used if necessary. More specifically, the first maximum value detection circuit can be a first detection circuit for selecting and detecting a specific signal from N signal voltages.
The photosensitive device of the present invention may be characterized by further comprising (1) N second integrating circuits which are arranged in correspondence with the N photosensitive elements, integrate charges in correspondence with the signal currents output from the photosensitive elements, and output signal voltages, (2) switches and capacitors inserted in turn between N sets of neighboring second and first integrating circuits, (3) a second maximum value detection circuit for detecting a maximum value of the signal voltages output from the N second integrating circuits, and (4) a timing control circuit for controlling operation timings of the first and second integrating circuits on the basis of the maximum value detected by the second maximum value detection circuit.
In this case, the photosensitive device comprises N sets of photosensitive elements, second integrating circuits, switches, capacitors, and first integrating circuits, which are connected in the order named. The capacitor and first integrating circuit of each set constitute a so-called CDS (Correlated Double Sampling) circuit. Each second integrating circuit receives a signal current which is output from each photosensitive element in correspondence with the intensity of incident light, integrates a charge based on the signal current, and outputs a signal voltage. The second maximum value detection circuit detects the maximum value of the signal voltages output from the N second integrating circuits. The timing control circuit controls the operation timings of the first and second integrating circuits on the basis of the maximum value detected by the second maximum value detection circuit. As a result, various noise components are removed from the signal voltages output from the second integrating circuits. Note that a signal to be selected and detected by the second maximum value detection circuit may be the second largest numerical value in place of the maximum value, or a numerical value of an appropriate order may be used if necessary. More specifically, the second maximum value detection circuit can be a second detection circuit for selecting and detecting a specific signal from N signal voltages.
The photosensitive device of the present invention is a photosensitive device used together with projection means for projecting spot light toward an object, and is also preferably characterized in that a timing control circuit (1) controls the N second integrating circuits to integrate first charge amounts on the basis of signal currents output from the N photosensitive elements upon receiving spot light components and background light components during a first period in which the projection means projects the spot light onto the object, and (2) then controls the N second integrating circuits to integrate second charge amounts on the basis of signal currents output from the N photosensitive elements upon receiving background light components and controls the N first integrating circuits to integrate charge amounts as differences between the first and second charge amounts during a second period in which the projection means does not project spot light onto the object. In this case, even when the background light components of incident light on each photosensitive element are larger than the spot light components, each first integrating circuit can obtain an image of only spot light components by subtracting the image sensing result of the background light components from that of the spot light components and background light components. A digital signal output from the A/D conversion circuit based on the spot light components obtained as the difference can have high resolution.